The main nutritional role of the placenta is to provide the correct mix of nutrients in sufficient quantities to support fetal growth and development throughout pregnancy. It has to do this whilst coping with wide variations in maternal nutrient intake between pregnancies and temporal variations within a pregnancy. The effective barrier to nutrients within the placenta is a single layer of cells called the syncytiotrophoblast, which close to term is around 4 mm thick with a total exchangeable surface area of around 10-15 m2. This membrane represents a nutritional 'bottleneck' where competition for nutrient transporters and metabolic selectivity allows the placenta to regulate the nutrient mix within the fetal circulation to one best suited to fetal development. In addition to its role as a simple nutrient transporter the placenta also acts as an
Whelton PK, He J, Cutler JA et al. (1997) Effects of oral potassium on blood pressure. Meta-analysis of randomized controlled clinical trials. Journal of the American Medical Association 277: 1624-1632.
extra fetal organ for many metabolic transformations with the feto-placental unit working as a metabolic whole. Adaptive mechanisms within the placenta allow the mother to meet the nutrient demands of the growing fetus whist consuming apparently poor diets during pregnancy. The fetus itself plays an active role in regulating key aspects of placental metabolism and nutrient transfer function to meet its own nutrient requirements.
Under normal circumstances the nutrient transfer capacity of the human placenta exceeds the fetal requirement and a considerable proportion of transport function would have to be lost before it became limiting for fetal growth. Although relatively rare, intrauterine growth restriction resulting from utero-placental insufficiency does occur. However, this is a complicated syndrome in which almost all aspects of placental and fetal metabolism are altered and it may not simply be due to a limitation of placental nutrient transfer capacity.
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